Chainstay length should not be an issue. Long stays were always common on most Touring bikes for many years and makes perfect sense - you don't want your heels to be continually kicking against full sized panniers or need to mount the bags too far aft on a rack. Perhaps the fishtailing issue on the Trek bikes was due to selection of too light weight tubing for use on a Touring bike - where you could expect a heavy load on the rear (the frameset was all Reynolds butted 531 tubing, I believe).

The broken stays on Surly Long Haul Truckers is inexcusable. Perhaps a design flaw or maybe just sloppy brazing? Surly may have resolved the issue now since it was apparently not an isolated incident. The frame I saw was replaced under warranty - the bike was nearly new - but what a disaster to have encountered when the owner was out touring along a country road when suddenly: SNAP!